The Trials And Tribulations Of Finding A Replacement Thermistor.

[KarlG]

So now that I have the necessary data to find a replacement thermistor I've spent almost two days browsing the IN. Boy am I having fun. 

I'll try and keep this as long as possible, oh! sorry short as possible.

Using the data that I've obtained I paid RS a visit and found a list of six possible thermistors. I tried to check the data sheets for the parameters pertaining to one of them and found four columns of data, 1006, 1008, 1009, and 1010, but no mention of which column was the correct one. So onto the phone to RS to find out which was the correct data sheet, after much who and ha and pumping data into the computer, plus a five minute wait to check with the tech dept, "sorry we can't help you". 15 minute call.

So into the IN and downloaded all the data covering these Thermistors, and this is what I found:

For any Part No. there a four variations, taking B57540G1103 as an example.
7003 B57540G1103+005 Dumet wires
7003 B57540G1103+007 Nickle-plated wires
8307 B57540G1103+000 Dumet wires
8307 B57540G1103+002 Nickle-plated wires

7003, and 8307 are the data sheets for the two beta variants. Eh! what ever happened to 1006 etc.? 

TDK (Epos) inform me RS has missed out the suffix from the Mfr. Part No.

After cross checking the data from TDK & Epos's data and RS's I couldn't find any correlation between the five charts.

Back to RS, "can you supply me with the suffix numbers for these items?". Reply after five minute wait "sorry we haven't got any suffixes". "Why are your data sheets incorrect?", five miuntes latter, "these are the data sheets provided by Epos, why don't you call TDK for help". "There's no point in calling TDK, they are not allowed to inform me as to what items are provided to a retailer or know why you have the wrong data sheets". "Once again sorry, I can only suggest you talk to TDK". My reply "Or better still, I go to another retailer, thank you I'll now go back to batting my head against the brick wall". 25 minute call.

They're coming to take me away ha ha, they're coming to take me away he he, to the happy farm were all is free.
For those of you who have never hear this, have a listen.
https://www.youtube.com/watch?v=hnzHtm1jhL4
or if you're like me.
https://www.youtube.com/watch?v=v9VMlLgc4vw

Sorry, but I must go now, there's two men in clean white coats hammering on my door.

[KarlG]

I should have poster this item first, but the frustration with RS prompted me to post those results first. Oh and bye the way those two men in their clean white coats were only there to clean the loo.

Thanks to Chas and Perran, I have received the following data.

From Perran:

My conclusions are as follows:

The thermistor has a nominal resistance of 10K ohms at 25C. The Beta value of the original thermistor was ~ 3500.
This defined the slope of resistance change versus temperature. The two resistance values of 1100 and 900 ohms therefore correspond to actual thermistor temperatures of ~ 93C and 101C.

The closest I can find in a readily available thermistor of the right size is from Farnell part no: 970-7271. It has a slightly different Beta value and will sound warnings at 87C and 93C.

I found data from Polymotor which indicated that a max temp of 150C would be OK so there are some big safety margins here.

So now I can correlate the temperature to required resistance. So I checked the data for the recommended replacement and came up with the following data. As you can see from the data sheet below 1100Ω corresponds to ~ 84°C and 900Ω ~ 90.5°C as apposed to the original values of ~ 93°C and 101°C.  For those of you who live in a hilly area this will be quite a negative effect. Seeing that the motor is good up to 150°C, I would not select this thermistor.

http://i1077.photobucket.com/albums/w477/XE521/Farnell_Data_zpsoygmph99.gif

The selection of thermistors is limited mainly by the physical size, to all intents and purposes the beta factor is not so critical. As I mentioned before I found a few at RS that can be used but due to the missing suffix code it's not possible to select one. I've pumped the Mfr. Part No. into the IN, but up till now the only suppliers I can find insist on an minimal order of 1000 pieces. I haven't finished the search yet, so with a bit of luck I'll come up with one.

There is a way to trick the circuit to obtain a better performance of the cutoff point, e.g. 120°C or so, this is still well within the safety margin. But I'll cover this subject at a later date when I find a suitable thermistor.